In conventionally used aqueous batteries, such as nickel hydrogen batteries and lead acid batteries, because of a restriction of the electrolyte voltage of water, voltage per cell unit has been limited to approximately 1.2 volts maximum. In recent years, there have been demands for portable equipment to be shrunk and natural energy power generation to be effectively utilized, so the necessity of using lithium ion batteries that are capable of handling higher voltage and having high energy density is increased. As packing material used for such lithium ion batteries, conventionally, metal cans have been used. However, considering a demand of thinner products and various requirements, a laminate packing material in which a resin film is laminated on an aluminum foil that has low cost when forming bag-like shapes has come to be used.
The secondary battery laminate packing material (hereinafter referred to as packing material) 10 is a laminate body constituted of metal foil and resin. As shown in FIG. 1, the packing material 10 is constituted by, in order, generally, an inner layer, an inner resin layer 11, an inner adhesive layer 12, a corrosion prevention processed layer 13, a barrier layer 14, a corrosion prevention processed layer 13, an outer adhesive layer 15 and an outer layer 16. As for the barrier layer 14, aluminum or stainless steel are used. As for the outer layer 16, a single layer film such as nylon or PET (polyethylene terephthalate) or a multilayer film is used. An electrode terminal which is called tab is required in order to supply power from the lithium ion battery constituted by the packing material 10. FIGS. 3A and 3B are cross-sectional views schematically illustrating the structure of the tab 20. The tab 20 is constituted by a metal terminal (hereinafter also referred to as a lead) 27 and a metal terminal coating resin film (hereinafter also referred to as a sealant) 24.
Aluminum is used for the lead 27 of the positive electrode and a corrosion prevention surface processing is often performed on the surface thereof. As a result of the corrosion prevention surface processing, an anticorrosion protective layer 25 is formed on the surface of the lead 27. Meanwhile, nickel or copper is used for the negative electrode of the lead 27. A single layer or multilayer resin film is generally used for the sealant 24. Since the sealant 24 is a member disposed between the lead 27 and the packing material 10, mainly the following three properties are required. The first one is having adhesive properties to both the lead 27 and the inner resin (resin that forms inner resin layer 11). Regarding adhesion to the lead 27, polypropylene or polyethylene which is a polyolefin resin used for the sealant 24 is acid-modified and a polar group is provided to enhance the adhesive properties.
Further, as shown at portion X in FIG. 3B, when the sealant 24 is welded to the lead 27, the end portion 27a of the lead 27 has to be filled up with melted sealant resin. If the filling is not sufficient, the lead 27 and the sealant 24 are not adhered. Hence, leakage of liquid or peeling may occur when the battery is manufactured. To enhance filling ability of the sealant 24 filling to the end portion 27a of the lead, it is important to make the MFR (melt flow rate) of the sealant resin larger so as to allow the sealant resin to easily flow when welding. For the above-described inner resin, a polyolefin type resin such as polypropylene or polyethylene is generally used. Therefore, polyolefin type resin can be used for the above-described sealant resin so as to obtain improved adhesive properties.
The second one is securing insulating properties. Since the lead 27 is a terminal from which current from the battery is acquired, insulation with other members should be maintained. In the tab member, a portion most concerned with insulation properties is, as shown at portion Y in FIG. 3B, a shoulder portion 27b of the lead 27. The lead shoulder portion 27b may break through the sealant 24 when there is a burr on the lead 27 so that the film thickness of the sealant 24 may become too thin if the pressure and temperature conditions are severe when the lead 27 and the sealant 24 are welded. Therefore, film thickness of the sealant resin of this portion (i.e., lead shoulder portion 27b) is most likely to become thin so that the insulation properties are likely to decrease. To solve the above-described problems, it is necessary to make it hard for the resin to flow when welding by using resin having a low MFR.
The third one is that a shape maintainability of the sealant 24. As shown at portion Z in FIG. 3B, the tab 20 has a portion constituted by only sealant resin. In this portion, depending on conditions of heating/cooling at the welding, undulation/bending may occur or sagging down due to the own weight after the welding (with cured while being deformed).
To try to obtain the above properties, a three-layer configuration using layers having different MFRs is proposed according to patent literatures 1 and 2 (as identified below). When welding, a highly fluidized bed that enables resin to easily flow and a low fluidized bed that makes it hard for resin to flow are laminated, whereby properties of the resin to wraparound towards the lead end portion 27a and insulation properties by maintaining the film thickness of the lead shoulder portion 27b are obtained. However, for the use of large capacitive batteries representing a vehicle-use or a storage battery, the film thickness of the lead 27 is likely to increase. Hence, a requirement for filling ability of the sealant 24 becomes more demanding. Moreover, these large batteries have large capacities so that the requirements of the insulation properties become demanding. Therefore, according to the configurations of the patent literatures 1 or 2, the performance thereof is not sufficient.